Why does editing a material on one object change lots of other object's materials as well?
The comments give the right answer (sorry to &quot;steal&quot; it, but otherwise they won't be marked as an answer). If you want an object to be the only user of a material you've given to multiple objects, click on the number next to the name. By default, the material called &quot;Material&quot; in your example will then be duplicated and renamed to &quot;Material.001&quot; and the active object will be the only user. Of course you can now assign this material to other objects as well if you want them to have the same material.
The advantage of having the same material on multiple objects is of course to be able to change materials, textures etc. on many different objects without having to merge them into a single object.
The advantage of duplicating a material by clicking on the number is, especially if you have a very complex node setup, that you can easily assign a material on one object that is similar to a different object and only make slight changes.